The .alpha.-thalassemia (".alpha.-thal") are common genetic disorders that result from reduced synthesis of the a globin chains of fetal (.alpha..sub.2.gamma..sub.2, HbF) and adult (.alpha..sub.2.beta..sub.2, HbA) hemoglobin (Weatherall and Clegg, 1981; Higgs et al., 1989, Higgs and Weatherall, 1993). The normal human .alpha. globin gene cluster is located on the short arm of chromosome 16 (Breuning et al., 1987; Buckle et al., 1988). In .alpha.-thal syndromes, .alpha.-globin synthesis is either diminished or absent due to either deletional or non-deletional abnormalities involving the .alpha.-globin genes (Higgs et al., 1989; Higgs and Weatherall, 1993). Diploid cells have four .alpha.-chain genes (i.e., .alpha..alpha./.alpha..alpha.). The severity of the hematological and clinical picture is directly proportional to the number of involved .alpha.-globin genes and thus the deletion of one, two, three, or all four of these .alpha. genes attribute to mild to complete a chain deficiencies syndromes.
The deletion of .alpha.-chain at birth results in the formation of a .gamma. chain tetramer, Hb Bart's (.gamma..sub.4). The percentage of Hb Bart's present corresponds to the degree of .alpha. chain deficiency (Cong & Shong, 1982; Liang et al., 1985; Higgs et al., 1989; Higgs and Weatherall, 1993). The .alpha.-thal-2 genotype has been found to have one of the two genes deleted, thug these heterozygotes (.alpha..alpha./-.alpha.) possess a mild .alpha. chain deficiency resulting from the presence of only three .alpha. chain genes and have not more than 2% Hb Bart's at birth (Higgs et al., 1989). Homozygotes (-.alpha./-.alpha.), as well as .alpha.-thal-1 heterozygotes (.alpha..alpha./--), possess moderate a chain deficiency resulting from the presence of only two .alpha. chain genes and have approximately 5% Hb Bart's at birth (Higgs and Weatherall, 1993). While Hb H disease is a heterozygosity for .alpha.-thal-2 in association with an .alpha.-thal-1 heterozygosity (--/-.alpha.), a severe .alpha. chain deficiency occurs due to the deletion of three .alpha. chain genes (Higgs et al., 1989; Higgs and Weatherall, 1993). The .alpha.-thal-1 homozygotes (--/--), which lack any functional .alpha. genes, present with Hb Bart's is known as hydrops fetalis syndrome and results in intra-uterine or early post-delivery death to the fetus (Lie-Injo & Hie, 1960; Higgs et al., 1989). Mother bearing fetuses with homozygous .alpha.-thal-1 (--/--) are at high risk for obstetrical complications, such pregnancy induced hypertension, eclampsia, and/or death.
At present, for a variety of technical, logistical and economical reasons, large-scale carrier screening and appropriate ante-natal detecting programs have not been established for the populations in which this syndrome occurs (Bowden et al., 1992; Higgs and Weatherall, 1993).
It is an object of the invention to develop a diagnostic method and kit for the detection and quantitation of hemoglobin (Hb) .alpha. gene(s) in .alpha.-thalassemia patients.
It is another object of the present invention to develop a method and kit for screening for carriers of the genetic disorder, .alpha. thalassemia.
It is yet another object of the present invention to identify persons who are at risk of having offspring with homozygous .alpha.-thalassemia as well as identify .alpha.-thalassemia in individuals with unexplained microcytosis and hypochromia.
Another object of the present invention is to develop a sensitive, non-radioisotopic test capable of differentiating between the various forms of .alpha.-thalassemia, by detecting and quantitating a gene(s) from blood samples.